The invention relates to a process for the preparation of copolymers of carbon monoxide and one or more ethylenically unsaturated compounds.
Linear copolymers of carbon monoxide with one or more ethylenically unsaturated compounds can be prepared by contacting the monomers at elevated temperature and pressure with a suitable catalyst, generally based on, inter alia, a Group VIII metal component, in the presence of a diluent in which the copolymers are insoluble or virtually insoluble. During the polymerization, the copolymers are obtained in the form of a suspension in the diluent.
In these copolymers the units originating from carbon monoxide and the units originating from the ethylenically unsaturated compound(s) occur in a substantially alternating arrangement.
The said copolymers have established utility in various outlets for thermoplastics. They may be processed by means of conventional techniques into films, sheets, plates, fibers and shaped articles such as containers for food and drinks and parts for the car industry.
A suitable preparation method, which is performed in batch operation, is described in EP-A-181014.
The batchwise preparation of the copolymers is generally carried out by introducing the catalyst into a reactor which contains the diluent and monomers and which is at the desired temperature and pressure. As the copolymerization proceeds the pressure drops, the concentration of the copolymers in the diluent increases and the viscosity of the suspension rises.
The reaction is discontinued when the viscosity of the suspension becomes very high, because of severe agglomeration of copolymer particles and continuing the process would create difficulties in view of poor heat transfer in the suspension and stirring problems.
A variant of batchwise copolymerization is the semibatchwise operation whereby besides the temperature, the pressure is also kept constant by adding monomers to the reactor during the copolymerization.
One of the important properties of the copolymers in question is the bulk density. This plays an important role both in the preparation and in the treatment, storage, transport and processing of the copolymers to be used in any of the aforesaid applications. As far as copolymer preparation is concerned, the rule of thumb is that the maximum permissible suspension concentration, expressed in (kg copolymer/kg suspension).times.100, is about one tenth of the bulk density expressed in kg/m.sup.3. This means that in the preparation of a copolymer with a bulk density of 100 kg/m.sup.3, the maximum suspension concentration is about 10%, whereas in the preparation of a copolymer with a bulk density of 200 kg/m.sup.3, the maximum suspension concentration is about 20%.
A doubling of bulk density offers the possibility of preparing about twice the quantity of copolymer in the same reactor volume. During copolymer treatment such as filtering, washing and drying, the quantity of adhering liquid is largely determined by the copolymer bulk density. Thus it has been found that a copolymer with a bulk density of 100 kg/m.sup.3 binds approximately 5 kg diluent or washing liquid per kg, whereas to a copolymer with a bulk density of 200 kg/m.sup.3 considerably less liquid is bound. This is of great importance because of the quantity of liquid to be used in washing the copolymers and which has to be removed when the copolymers are dried. As far as transport and storage are concerned, the rule is that the higher the copolymers' bulk density, the more attractive the flow behavior they will have and the less space they will occupy. As regards the processing of the copolymers into shaped objects, it is often the case that copolymers with low bulk density cause problems in the processing apparatus. For copolymers with low bulk density, e.g. of below 200 kg/m.sup.3, a compacting treatment, for example by extrusion, in order to make them suitable for further processing in the usual equipment for that purpose, could cause a problem. The higher the copolymers' bulk density, the easier the prior treatment of the material, particularly extrusion.
It will be clear from the above that the main interest is in a process whereby a relatively high suspension concentration is achieved and whereby copolymers having a high bulk density are produced.
However, a complication is formed by the above-mentioned agglomeration of copolymer particles, which may occur, before the suspension concentration has reached a value required for obtaining copolymers in an adequate yield.
An investigation of this phenomenon has revealed that the onset of severe agglomeration can be postponed by increasing the amount of catalyst used. However, when operating the process on a commercial scale, for economic reasons, the use of larger quantities of catalyst is undesirable.
It has further been proposed to prepare copolymers having a high bulk density by supplying 25 to 85% of the total amount of the catalyst composition at the outset of the reaction and the remaining part of the catalyst composition at a later stage in the process. However, at the obtainable suspension concentrations, the copolymer yields leave room for improvement.